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As someone not from the US I would say one of the reasons Everyone talks about Brave New World and Orwells works are because, while they are set in a future US and UK the setting is not that relevant and could be "Any western democracy at some time in the future" while “It Can’t Happen Here” is very much set in the USA and at a certain time.
Why it is not more popular in the USA?
My guess is that during the cold war, anything that specific about fascism in the USA would have not been allowed to be widely used in US schools.